1. Field of the Invention
The present invention relates to a magnetic head using a specific sealing glass for assembling.
2. Description of the Related Art
With the progress of electronics, the need for various kinds of glass has been increased.
The use of a sealing glass will be explained by making reference to a sealing glass for a magnetic head. FIGS. 1A to 1K illustrate a typical process for producing a magnetic head.
First, a piece of ferrite is cut out from an ingot of ferrite (FIG. 1A). The piece of ferrite is abrased to form a rod having a suitable shape and size (FIG. 1B). As shown in FIG. 1C, grooves are formed by track processing. Over the grooves, a glass is molded (FIG. 1D) and excessive glass is removed by abrasing. After forming a groove for coiling, a gap surface is abrased (FIG. 1E). Then, a gap glass is sputtered on a bonding surface and a pair of ferrite pieces are bonded to form a gap (FIG. 1F). The bonded pieces are cut to form a chip (FIG. 1G) and sides of the chip are abrased to a suitable thickness (FIG. 1H). The chip is then adhered to a base (FIG. 1I), and a tape flying face is abrased (FIG. 1J). Finally, a wire is wound to finish a magnetic head (FIG. 1K).
As a material for the magnetic head, the ferrite has been most widely used. However, since the ferrite has a small saturation magnetic flux density, the magnetic head comprising the ferrite cannot be used in combination with a a high density recording medium such as a metal tape which has recently been developed because of magnetic saturation of the magnetic head. Then, as a substitute magnetic material for the ferrite, an amorphous alloy was developed and used for the production of a magnetic head which is assembled in a VTR used in broad casing stations or an S-VHS type VTR. For producing the magnetic head comprising the amorphous alloy, the amorphous alloy is sputtered between the steps of FIGS. 1B and 1C. However, since the amorphous alloy tends to be easily crystallized, a working temperature of the sealing glass used for assembling the magnetic head should be 500.degree. C. or lower. To this end, a sealing glass containing a very large amount of PbO is used. In general such glass has small strength and poor water resistance.
Then, a superstructural nitride alloy as a substitute for the amorphous alloy was invented (see, for example, the Technical Study Reports of the Electronics, Information and Communication Society, MR-86-4, 87-14). Since the superstructural nitride alloy has a coefficient of thermal expansion of 105.+-.5.times.10.sup.-7 /.degree. C. (30.degree. to 300.degree. C.) and a crystallization temperature of 600.degree. C. or higher, the sealing glass containing a smaller amount of PbO can be used. Therefore, the strength and water resistance of the sealing glass are expected to be improved.
However, conventional glasses which can be processed at a temperature of 600.degree. C. or higher are not suitable as sealing glasses for the magnetic head. For example, Japanese Patent Kokai Publication No. 501372/1983 discloses a glass which contains 62 to 72% by weight of PbO and has a working temperature of 530.degree. C. But, its coefficient of thermal expansion is as small as 87.times.10.sup.-7 /.degree. C., it has poor water resistance because of the presence of PbF.sub.2, it is easily devitrified because of the presence of TiO.sub.2, and it is colored in dark brown because of the presence of V.sub.2 O.sub.5.
Japanese Patent Kokai Publication No. 146951/1984 discloses a glass which contains 70 to 74% by weight of PbO, and has a viscosity of 10.sup.6 poises at 600.degree. C. or lower and a coefficient of thermal expansion of 83 to 93.times.10.sup.-7 /.degree. C. Since, in general, the sealing glass should have a viscosity in the order of 10.sup.4 poises at the working temperature, the working temperature of this glass exceeds 600.degree. C. In addition, the coefficient of thermal expansion is small.
When a coefficient of thermal expansion of the sealing glass is too small, a large tensile stress is generated in the substrate so that the chip is broken in the step (g) of the above process. When the coefficient of thermal expansion is too large a large tensile stress is generated in the glass so that the glass is broken.
When the glass has prior water resistance, the surface of the glass is denatured and the gap formation is insufficient in the step (f), so that the chip is broken in the step (g).
When the glass is colored or devitrified, the apex is not observed when the gap depth is adjusted to a defined depth.
When the working temperature exceeds 600.degree. C., a super structure nitrogen alloy such as a Co-Nb-Zr-N alloy which is used as a magnetic material of the magnetic head is crystallized so that the magnetic head does not work.
Japanese Patent Kokai Publication No. 195552/1984 discloses a glass for thick coating a circuit which contains 65 to 75% by weight of PbO, and has a baking temperature of 560.degree. C. or lower and a coefficient of thermal expansion of 65.degree. to 85.times.10.sup.-7 /.degree. C. This glass has the small coefficient of thermal expansion and is colored in dark green because of the presence of Cr.sub.2 O.sub.3.
Japanese Patent Kokai Publication No. 170240/1988 discloses a glass which contains 45 to 60% by weight of PbO, and has a working temperature of 550.degree. C. and a coefficient of thermal expansion of 119 to 126.times.10.sup.-7 /.degree. C. Since this glass contains a comparatively large amount (5% by weight or more) of Na.sub.2 O, it has poor water resistance. In addition, its coefficient of thermal expansion is too large.
In view of the above state of the arts, the sealing glass to be used together with the superstructural nitride alloy should meet at least following requirements:
(1) Its coefficient of thermal expansion is close to that of the superstructural nitride alloy. PA0 (2) Its working temperature is not higher than 600.degree. C. PA0 (3) It has good water resistance. PA0 (4) It is not darkly colored and not devitrified during processing. PA0 10 to 25% by weight of SiO.sub.2, PA0 55 to 80% by weight of PbO, PA0 1 to 15% by weight of CdO, PA0 0 to 10% by weight of B.sub.2 O.sub.3, PA0 0 to 5% by weight of ZnO, PA0 0 to 5% by weight of Al.sub.2 O.sub.3, PA0 0 to 5% by weight of Na.sub.2 O, and PA0 0 to 10% by weight of K.sub.2 O PA0 10 to 20% by weight of SiO.sub.2, PA0 60 to 75% by weight of PbO, PA0 1 to 10% by weight of CdO, PA0 3 to 10% by weight of B.sub.2 O.sub.3, PA0 0 to 5% by weight of ZnO, PA0 0 to 3% by weight of Al.sub.2 O.sub.3, PA0 0 to 3% by weight of Na.sub.2 O, and PA0 1 to 5% by weight of K.sub.2 O
As a result of preliminary experiments on the coefficient of thermal expansion and water resistance, it was found that the glass having a coefficient of thermal expansion of from 90 to 110.times.10.sup.-7 /.degree. C. has good sealing properties with the superstructural nitride alloy, and that the glass should have the water resistance of at least "Second Class", preferably "First Class" when expressed according to the classes determined by the Japan Optical Glass Industries Association (Nippon Kogaku Garasu Kogyokai).